Municipal jurisdictions have building codes and construction regulations that set standards for proper installation of electrical power and distribution circuits in all structures, including residences and industrial and commercial premises. Most jurisdictions require visual inspection of electrical installations before power may be turned on for a building. Some jurisdictions, such as in Europe, Australia, and New Zealand, require an actual test before power is connected to a building, more tests after power is connected, and annual tests of all circuits to ensure the safety of the electrical service to the building.
For new installations, inspectors will perform one or more tests before power is connected. These tests include insulation testing and ground testing. Each circuit is tested to see if its resistance is proper. A 500 V DC voltage is applied to each circuit and the resistance is measured. A short circuit or a breakdown or degradation of the wiring insulation will be detected by a voltage drop. Such short circuits may occur, for example, due to a nail piercing a conductor and shorting one phase line to another phase line. Another pre-power test is a test made on the electrical ground of the building.
If the electrical distribution system of the building passes the pre-power tests, power is connected and the inspector performs another series of tests, such as a residual current detection (RCD) test and a loop impedance test. In Europe, circuit breakers are equipped with ground fault detection and interruption circuits. Before a new building may be occupied, each breaker is individually tested to insure it is properly working. In commercial and industrial buildings, the circuit breakers are often located in one room. As such, the breaker room may have hundreds of circuit breakers, each of which is individually tested. After completing the circuit breaker tests, the inspector performs a loop impedance test on each outlet.
RCD Test
An RCD test detects fault currents flowing to earth (ground) that are too small to trip over-current protection devices. In other words, a fuse or circuit breaker designed for a particular current and voltage may not detect a lower but faulty current that is still sufficient to cause electrical shock or an electrical fire. Basic testing of RCDs involves determining the tripping time (in milliseconds) by inducing a fault current in the circuit.
Loop Impedance Test
Earth loop impedance testing is essential to discover whether a live conductor is accidentally connected to an earth conductor in a faulty appliance or circuit. If so, the resulting short-circuit current to earth may be high enough to cause electric shock or generate enough heat to start a fire. Normally, the fuse will blow or another circuit protection device will trip, but a situation may arise where the actual short-circuit current in a faulty installation is of insufficient level and the protection device would thus take too long to activate. The delay can be disastrous for life and property. It is therefore necessary to know if the impedance of the path that any fault current would take is low enough to allow sufficient current to flow in the event of a fault and that any installed protective device will operate within a safe time limit.
A loop test is performed on each outlet. The test requires inserting a known, low resistance between one phase line and the neutral line and measuring the voltage drop across the known resistor, which in turn gives a measure of the total loop resistance. The resistance of the known resistor is subtracted from the total resistance to give the resistance of the fault loop. The duration of the test must be limited to one or two cycles of the voltage or else the inserted known resistance may trip the circuit breaker.
Multifunction Tester
The foregoing pre-power and power-on tests are normally performed by a technician using a multifunction test device, including, and not limited to, the 1650 Series Multifunction Installation Testers made by Fluke Corporation. Those installation testers have an oblong body with a central display, a rotary dial knob on the right side of the display that indicates clearly which test is selected, and on the left-hand side, functional switches to set one or more parameters for the selected test. The installation testers have a padded strap that the technician places around his neck. The top of the display has input jacks for three input probes or a probe cord with a three-prong receptacle.
To perform an RCD test on a breaker with a rated current of 30 ma, an input probe from one jack is temporarily touched to one side of a breaker and a current of 30 ma is injected into the breaker. The technician holds the probe against a breaker terminal with one hand, and with the other hand, touches or flips an RCD test switch to apply 30 ma from the tester to the circuit breaker. The display indicates the test results, which the technician may save in tester memory. The foregoing procedure is repeated for each circuit breaker.
Although 30 ma is a low current, the repetition of tests on multiple circuit breakers generates heat. At times, the installation device may become too hot to operate and must be allowed to cool down. The interruption of RCD testing reduces the efficiency of the overall testing operation. It is also possible the technician may be distracted and slip the probe off the circuit breaker when he looks at the display to confirm the reading. It is possible the probe could short out one or more breakers when it slips.
To perform loop testing, the technician locates each outlet, unplugs any equipment at the outlet, and runs a loop resistance test. The outlet may be obstructed by equipment or furniture. Nevertheless, the technician, with the tester hanging around his neck, plugs a three-prong test cord into the outlet. The other end of the test cord is plugged into the tester. The technician takes readings of the voltage at the outlet pre-test and then presses a loop test button to perform the loop resistance test. The results of the test may be stored in the tester memory. One can appreciate that even when using a lightweight tester, it is nevertheless cumbersome for a technician to position the display close enough to each outlet to run the loop test.